Air Venturi Avenge x reg pressure not consistent

When I fill up my Avenge X to 4000psi/300bar the reg (factory setting) creeps up just above 2000-2100 psi. After 10-20 shots it lowers itself to around 1800psi. Is this normal or is there something wrong with the reg pressure system? Also I have a hard time grouping my shots well when shooting 40+ meters. Shooting Crossman Domed Ultra Magnum 10.5gr
Thanks - I am still new and learning🎃